The present invention generally relates to medical imaging software, methods, and databases. Specifically, the present invention relates to improving machine-to-machine and temporal (e.g., inter and intra-machine) consistency of coronary calcium scoring by using an algorithm that can either sharpen or smooth the image, so as to return the spatial resolution of the processed image to a certain reference value.
Coronary artery calcium quantitation is a major focus in the effort to assess risk for coronary heart disease, to monitor progression of plaque development, and to potentially assess therapies and interventions designed to reduce mortality from coronary heart disease. (See Rumberger J. A. et al, “Electron Beam Computed Tomographic Coronary Calcium Scanning: A Review and Guidelines for Use in Asymptomatic Persons,” Mayo Clinic Proc. 1999; 74:243–252 and Schmermund A., et al, “An Algorithm for Noninvasive Identification of Angiographic Three-Vessel and/or left Main Coronary Artery Disease in Symptomatic Patients,” J. Am. Coll. Cardiology 1999; 33:444–452, the complete disclosure of which are incorporated herein by reference). Although the current orthodoxy is that the rupture of soft plaque and subsequent thrombus formation is the major precursor of acute coronary events, in most individuals it is believed that coronary calcium burden is also a valid surrogate or indicator of total plaque burden, including soft plaque.
The assessment of risk from coronary calcium is generally a two-step process: First, calcium burden is quantitated by a “scoring” algorithm, most commonly with the Agatston scale. (See Agatston A. S. et al, “Quantification of Coronary Artery Calcium Using Ultrafast Computed Tomography,” J. Am. Coll. Cardiology 1990; 15:827–832, the complete disclosure of which is incorporated herein by reference). Second, the measured calcium burden, age, and gender of the individual are used to rank the individual against his or her age-matched cohort.
X-rays in the energy range used for radiography are highly sensitive to the presence of vascular and extra-vascular calcium in the coronary arteries. Computed tomography (CT), with its ability to provide thin sections and three-dimensional slice registration, increases the sensitivity to the presence of small amounts of calcium in the patient's vasculature. Interestingly enough, the voxel volume of CT and digital radiography are not dramatically different. Compared to CT, radiography has much better in-plane resolution but much poorer resolution along the beam. The complexity of structures seen in a plane projection, however, makes the task of quantitating coronary calcium in a radiograph difficult.
Today, thousands of CT scanners perform coronary calcium scoring measurements and thereafter refer the measurements to a database. For example, a database of over 35,000 subjects has been accumulated by the University of Illinois at Chicago. See Hoff J. A. et al., “Age and Gender Distributions of Coronary Artery Calcium Detected by Electron Beam Tomography in 35,246 Adults,” Am. J. Cardiology 2001; 87:1335–1339, the complete disclosure of which is incorporated herein by reference. Calcium scores (e.g., Agatston Score or volume score) from an individual subject are compared to this database on the basis of age and gender of the patient. Most of the subjects in the existing database are Caucasian and middle to high socio-economic status, as well as asymptomatic, more males than females, and in the age range of 40–70 years of age. Because of the inherent differences in individual CT scanners and operators, there is considerable controversy as to whether calcium scores from different CT units can be compared to the established databases.
The controversy generally focuses on technology issues (e.g., the differences between helical CT scanners and electron beam CT scanners). Much attention was paid to time resolution and scanner calibration. Scanner calibration assures the HU scale and is well handled by the calibration procedures implemented by manufacturers. Blurring and motion artifacts due to heart motion can either increase or decrease the calcium score, depending on the particulars of the lesion and its motion.
Such a controversy, however, fails to account for operational parameters, machine maintenance, population variability, and spatial resolution. For lesions that have a dimension even a few times larger than the full-width-half-maximum (FWHM) of the scanner point spread function (PSF), both peak intensity and the apparent area of the lesion will be affected by the imaging device's spatial resolution. The spatial resolution of a scanner will depend on its x-ray source and detector configuration, as well as on reconstruction kernels and filters. In addition, the spatial resolution will change as the x-ray source ages.
Unfortunately, there is no process today to monitor and correct for temporal differences or changes in CT scanners, differences in scanner operating settings, or design factors, which vary among manufacturers and operators. Therefore, both the usefulness and quality of the databases, and the relationship of a particular individual to the population in the database may be degraded by uncertainties in the spatial resolution with which the database and the individual subject images were acquired.
Consequently, what is needed are methods and software that can improve the coronary calcium scoring consistency between a plurality of imaging machines or during the lifetime of a machine.